SELF. (VFO). The transmitter has two-bands over which it can work. On band 1 there are three choices of SPOT Frequencies which are colour coded: A - Blue. B - White. C - Yellow. On band 2, three more choices: C - Yellow (common to bands 1 and 2). D - Black. E - Brown. A transmitter can have either but not noth. The transmitter shown below in the photograph section is tuned to operate on C, D and E spot frequencies only.

(The apparent marking before 'A' and after 'C' are screwheads).

whereas another set will be prepared with a tuning dial showing the spot frequencies A, B and C. The frequencies are NEVER mentioned either to the operators of the set or to the planners of the communication circuits - all that is spoken/written of is the COLOUR/SPOT FREQUENCY LETTER. The set is tuned by a special test set ashore or in the HQ ship before issue for use. The receiver tuning dial allows the operator to tune across the whole frequency range of the receiver which covers A, B, C, D, E frequencies. These latter frequencies are shown by a continuous heavy black line marked on the RX tuning dial.

(the two apparent markings above the tuning scale are screwheads).

The best way to describe the alignment of the 686 transmitter is by using this block diagram of the system.

Here you can see the Test Set and plugged into that, are the earphones and an output from the 686's variable frequency oscillator (VFO). The test set generates harmonics of the 3Mc/s crystal, which, with the exception of the fundamental frequency, are fed to a mixer stage. The output of the mixer contains frequencies which are the sums and difference of the two inputs, plus the input frequencies themselves. The low pass filter feeds the difference to the AF amplifier and the zero beats can be heard when the crystal oscillator coincides with the free running LC oscillator.

WHERE USED/FITTED

Type 686. The set is not fully tropicalised. This set is used for communications between Combined Operations craft over distances of 2-10 miles according to whether R/T or MCW is employed, and also depending upon the maximum height at which aerials can be fixed in the craft concerned. The set, which is transportable, has been designed for operation by unskilled personnel. To ensure efficient operation it is most essential that adjustments of the set before issue for service should be correctly carried out. It has two types of aerial supplied [a] a dipole for omnidirectional working, and [b] a 4-element array for unidirectional working. The dimensions are 11" x 10" x 15.5". The battery accommodation is a further 12.5" x 6.875" x 9.75". The weights are: Set = 44lbs : Dipole Box = 24lbs : Battery = 61lbs : Array Box = 20lbs.